1. Field of the Invention
The present invention relates to a radiation generating tube which uses a transmissive target and is applicable to a radiation generating apparatus.
2. Description of the Related Art
A transmissive radiation generating tube generates radiation by accelerating electrons emitted from an electron emission source of a cathode with a high voltage applied between an anode and the cathode and irradiating a metallic target provided at the anode with the accelerated electrons, and is adopted in medical and industrial radiation generating apparatuses.
With such a radiation generating tube, voltage withstand capability have been an issue that makes downsizing and weight reduction difficult. Japanese Patent Application Laid-open No. H09-180660 discloses improving voltage withstand capability of a transmissive radiation generating tube by using a structure in which a focusing electrode of an electron gun is sandwiched between and fixed by an insulating tube and a cathode and in which a gap is provided between a tube wall and the focusing electrode in order to increase an insulation creepage distance of the tube wall. In addition, Japanese Patent Application Laid-open No. 2006-019223 discloses a reflective radiation generating tube in which irregularities with an arithmetic-mean roughness of 1 to 10 μm are formed on a vacuum-side surface of a glass insulator that supports a conductor in a vacuum chamber over a certain range from an end position of the conductor.
The following problem arises when attempting to achieve higher voltage or further downsizing of a radiation generating tube.
With a radiation generating tube in which a cathode is bonded to an end edge of an insulating tube, there is a structural risk that unintended electron emission may occur from a junction (bonded interface) between the insulating tube and the cathode. When increasing voltage or reducing a size of the radiation generating tube, electrons emitted from the junction may increase due to an increase in field intensity in a vicinity of the junction. Such emitted electrodes may electrically charge an inner wall of the insulating tube and may potentially cause a discharge.